The humidity-measuring instrument associated with Leonardo da Vinci’s designs is the hygrometer. His concept used the changing weight of an absorbent material to indicate the amount of moisture in the air.
Leonardo was interested in atmospheric conditions because they affected weather, materials, and the performance of proposed flying machines. An absorbent substance becomes heavier as it takes up water from humid air, providing a measurable change that can be compared against a scale.
The history of the hygrometer includes several early experiments, and attribution is not always as simple as naming one inventor. Leonardo’s notebooks contain an important Renaissance design, but later instruments made humidity readings more precise. Modern hygrometers may use electronic sensors, hair, polymers, or chilled mirrors. A hygrometer should not be confused with an anemometer, which measures wind rather than moisture.